CMOS image sensors may generate inaccurate image data due to dark current in the pixels themselves and variation in the level of dark current from pixel to pixel. Dark currents add to the output voltages and degrade the picture provided by the imaging system. Therefore, to generate accurate image data, it is desirable to reduce or eliminate dark currents. For a BSI CMOS image sensor, dark currents may be a particular problem. A typical BSI CMOS image sensor has dark current levels that are over 100 times greater than that of a front side illuminated sensor. This may be caused by the fabrication process to produce the relatively thin substrate layer of the BSI CMOS image sensor.
The fabrication process for BSI CMOS image sensors produces a relatively thin (e.g., less than 4 micrometers) backside silicon layer with techniques such as chemical mechanical polishing (“CMP”) and chemical etching. The resulting backside silicon surface may suffer from a high number of defects that may exacerbate dark currents by providing leakage paths, resulting in a relatively high number of hot pixels. This surface defect problem may be addressed by ion implantation of P or N type dopants into the backside surface. Backside dopant implantation produces fields that may facilitate the movement of photo generated electric carriers away from the backside surface. For example, for an N type photodiode, the photoelectric effect produces electrons as charge carriers. Therefore, a P type dopant may be implanted on the backside surface to generate a requisite field to reduce the contribution of the electron carriers from the backside surface into the phododiode. For a P type photodiode, the photoelectric effect produces holes as charge carriers. Therefore, an N type dopant may be implanted on the backside surface to generate the requisite field to reduce the contribution of the hole carriers from the backside surface into phododiode.